The Berlin U-Bahn originated in 1880 with Werner Siemens' idea to build an urban railway in Berlin. During the nine years after the German Empire was founded, the city's population grew by over one-third and traffic problems increased. In 1896, Siemens & Halske began to construct the first stretch of overhead railway. On 1 April 1897, the company began construction of an electric underground railway. The Berliner Verkehrs Aktiengesellschaft (BVG) was formed in 1928, and took over further construction and operation of the network. In 1938, the company was renamed Berlin Transport Company; the original acronym, however, remained. Since 1994, the BVG has been a public company.

The line between Stralauer Tor and Potsdamer Platz (the present-day U1 line) was the first to open, on 18 February 1902. Four additional lines were built before the First World War. In the Weimar Republic, hyperinflation initially prevented further expansion of the system. A new line with a wider car, known as Großprofil, opened in 1923.

No stations were built during the Nazi era, and many were destroyed in air raids during the Second World War. Near the end of the Battle of Berlin in early May 1945, the north–south tunnel was demolished and broad sections of the subway flooded; reconstruction of the prewar system lasted until 1950.

Construction of the Berlin Wall in August 1961 introduced restrictions. Lines C (the present-day U6) and D (U8) ran through tunnels and stations in East Berlin without stopping, except for the border crossing at Friedrichstraße station. The Warschauer Straße and Potsdamer Platz stations were decommissioned. Although U-Bahn construction continued in West Berlin, the Line E Tierpark station was the only one to open in East Berlin (in 1973). From 1985 to 1989, Line E (U5) was extended from Tierpark to Hönow.

On 11 November 1989, two days after the fall of the Berlin Wall, the first formerly closed station (Jannowitzbrücke) was reopened as a border crossing. Rosenthaler Platz followed on 22 December, and Bernauer Straße on 12 April 1990. All three stations are on the U8 line. On 1 July 1990, all remaining ghost stations were reopened. With the reunification of the system, U2 reopened in 1993. In 1995, the U1 was reinstated from Kreuzberg via the Oberbaumbrücke to Friedrichshain at Warschauer Straße station. The U2 Mendelssohn-Bartholdy-Park station opened in October 1998, and a short portion from Vinetastraße to Pankow opened in September 2000. Due to budgetary constraints, there has been no further expansion.

The U-Bahn was built in three major phases: Before 1913 – construction of the ("small profile") network in Berlin, Charlottenburg, Schöneberg, and Wilmersdorf 1913–1930 – introduction of the ("large profile") network, with the first north–south lines After 1953 – postwar development

Phase 1

Early lines

At the end of the 19th century, Berlin planners were looking for solutions to the city's increasing traffic problems. Industrialist and inventor Werner von Siemens suggested the construction of elevated railways, and AEG proposed an underground system. Berlin's administrators feared that an underground metro would damage the sewers, therefore favoured an elevated railway which would follow the path of the former city walls. However, the neighbouring city of Charlottenburg opposed an elevated railway running along Tauentzienstraße. After years of negotiations, construction began on a mainly elevated railway line on 10 September 1896. The railway would run between Stralauer Tor and Zoologischer Garten, with a short spur to Potsdamer Platz. Known as the , the line opened on 15 February 1902 and was immediately popular. Before the year ended, the line was extended: east to Warschauer Brücke (Warschauer Straße) by 17 August, and west to Knie (Ernst-Reuter-Platz) by 14 December. Construction of the Charlottenburg section began on 1 January 1900.

In 1902, the lines were: K1: Warschauer Brücke–(Gleisdreieck–Potsdamer Platz) K2: Warschauer Brücke–(Gleisdreieck–Zoologischer Garten) K3: Potsdamer Platz–(Gleisdreieck–Zoologischer Garten)

Charlottenburg extension After the original line opened, extensions were planned: from Charlottenburg to Wilhelmplatz (present-day Richard-Wagner-Platz) via Reichskanzlerplatz (Theodor-Heuss Platz), and from Potsdamer Platz in central Berlin along the Warsaw road to Frankfurter Tor. Negotiations with the developing Charlottenburg were completed quickly, since considerable vacant land could be developed. The most important negotiating point was a path for Rathaus Charlottenburg on the Wilhelmsplatz. An extension along the Berliner Straße (today's Otto-Suhr-Allee) was proposed, but there was concern that having the Berlin-Charlottenburg tram alongside parallel traffic was not sensible. The route under the Bismarckstraße would be extended further west and curve towards the town hall. The station was initially named Bismarckstraße (present-day Deutsche Oper), and the line between Knie and Wilhelmplatz opened on 14 May 1906. Both metro lines now extended to Bismarckstraße, but only the branch to Potsdamer Platz also went to the Wilhelmsplatz.

Whilst this route was under construction, the overhead railway company and the city of Charlottenburg agreed on a route to Berlin-Westend. The planned Bismarckstraße station had to be changed, since the line was to branch off there towards Westend. The station was Germany's first four-track metro station.

For the route westward to Charlottenburg, the following stations were to be built: Bismarckstraße (present-day Deutsche Oper) Sophie-Charlotte-Platz Kaiserdamm Reichskanzlerplatz (Theodor-Heuss-Platz)

Emperor Wilhelm II opened the line on 16 March 1908, and the inaugural trip became known as the "minister ride". It was opened to the public on 29 March 1908. The two existing lines ended at Reichskanzlerplatz, and a third line ran between Bismarck Street and Reichskanzlerplatz.

Deutsches Stadion was opened on 8 June 1913 as a terminus. The corresponding Grunewald station was completed in January 1913. Built to supply power to the metro, the nearby Unterspreewald power plant in Ruhleben began operation in 1911.

Because of the expanded route length, the workshop in Rudolfstraße (Warschauer Brücke) was too small; a larger workshop was needed. Since the Prussian forest administration wanted to sell their portion of the Grunewald profitably, the overhead railway company purchased 14 hectares of land to build their new operating workshop and began an extension to the stadium station (Olympia-Stadion). The company received ℳ200,000 from the Forestry Administration, and service was irregular.

The planned eastern extension of the main line from Warschauer Brücke to Frankfurter Tor was not built. Operated as a tram from Warschauer Brücke to the Central-Viehhof metro station, it was acquired in 1909 by the city and opened on 1 January 1910.

Eastern extension to Spittelmarkt In Berlin's city center, Potsdamer Platz was replaced by Leipziger Platz (Potsdamer Platz) to allow an extension to the line. Construction began on 15 December 1905. The line under Leipziger Straße to Spittelmarkt opened in 1908.

The 1908 lines included: K1: Warschauer Brücke – Potsdamer Platz – Spittelmarkt K3: Wilhelmplatz (Charlottenburg) – Wittenbergplatz – Potsdamer Platz – Spittelmarkt

The system was extended to Alexanderplatz by July 1913, with Wilhelmplatz–Alexanderplatz becoming the U-Bahn's busiest line. Three and a half weeks later, on 27 July, the northern extension to Schönhauser Allee (originally called Nordring) and the S-Bahn was opened.

Schöneberg line

Schöneberg also wanted a connection to Berlin. The elevated railway company did not believe such a line would be profitable, so the city built Germany's first locally financed underground. Running below the surface from Hauptstraße, the 2.9-km (1.8-mi) line needed a second, underground station at its Nollendorfplatz terminus because the existing station was part of the elevated railway. Construction began on 8 December 1908, and the line opened on 1 December 1910.

Dahlem and Kurfürstendamm lines

In the summer of 1907, Wilmersdorf's elevated railway company suggested an underground line to Nürnberger Platz and (if the new city would pay for it) Breitenbachplatz. Since Wilmersdorf had poor transport connections, its government took up the suggestion. Dahlem, south of Wilmersdorf and undeveloped, also supported a U-bahn line from Breitenbachplatz to Thielplatz.

The line would include the city of Charlottenburg, which saw Wilmersdorf as a competitor for affluent taxpayers. After long negotiations, an additional line under the Kurfürstendamm to Uhlandstraße was agreed in the summer of 1910 and work began on both lines. The double-track Wittenbergplatz station, with two side platforms, had to be rebuilt. The new station required five platforms, with a sixth for expansion and an entrance hall. Wilmersdorf and Charlottenburg submitted many design suggestions. Alfred Grenander, house architect of the elevated railway company, was appointed to design the station.

Although the line to Uhlandstraße (which branched at Wittenbergplatz and had no other stations) was intended to extend to Halensee, its only addition was the 1961 Kurfürstendamm interchange with the U9. An extension to the west is still planned, and preparations were made with the construction of Internationales Congress Centrum on Neuen Kantstraße. Provisions have also been made for a U1 station under the U7 station at Adenauerplatz. The Kurfürstendamm and Dahlem lines, which opened on 12 October 1913, were the last U-Bahn sections built before World War I.

The Dahlem line stations are: Wittenbergplatz (awaiting extension) Nürnberger Platz (closed in 1959 and replaced by Spichernstraße) Hohenzollernplatz Fehrbelliner Platz Heidelberger Platz Rüdesheimer Platz Breitenbachplatz (originally planned as Rastatter Platz) Podbielskiallee Dahlem-Dorf Thielplatz (preliminary terminus)

Gleisdreieck junction

One of the U-Bahn's most dangerous places was the triangular junction at Gleisdreieck, which connected the main line from Warschauer Brücke and Zoologischer Garten with the branch line to Potsdamer Platz. The branch line was protected only by signals, and train-driver inattention could be disastrous. On 26 September 1908, a U-Bahn train ran into the side of another train, forcing two carriages off the track; one fell off a viaduct, killing 21 passengers. As a result, it was decided to change the junction's configuration.

Construction began in May 1912 to replace the rail triangle with two lines at a grade-separated cross, with a new Gleisdreieck interchange station; these lines now form part of U1 and U2. Construction largely preserved service, with brief interruptions. On 3 November 1912, the new Gleisdreieck station opened; construction was completed in August 1913. The connecting track from Pankow to Warschauer Straße continued in use for supply vehicles during construction.

The train services from 12 October 1913 were organised as follows: Zuggruppe I: Nordring – Alexanderplatz – Gleisdreieck – Wittenbergplatz – Bismarckstraße – Wilhelmplatz (extended to Stadion when required) Zuggruppe II: Nordring – Alexanderplatz – Gleisdreieck – Wittenbergplatz – Fehrbelliner Platz Zuggruppe III: Warschauer Brücke – Hallesches Tor – Gleisdreieck Zuggruppe IV: Nollendorfplatz – Bayerischer Platz – Hauptstraße (Schöneberger Untergrundbahn) Zuggruppe V: Bismarckstraße – Reichskanzlerplatz (extended to Stadion when required) Zuggruppe VI: Fehrbelliner Platz – Breitenbachplatz – Thielplatz Zuggruppe VII: Wittenbergplatz – Uhlandstraße (Kurfürstendamm line)

Phase 2

Nord-Süd Bahn